Abstract
PURPOSE: A few years ago, we started to use a new freeze-thaw protocol for the frozen embryo transfer cycles. Instead of thawing the embryos 2-4 h prior to the transfer, we started thawing the embryos 20-22 h prior to the transfer. The aim of this study was to compare the pregnancy rate in cases of embryos that continued to develop in the post-thawing culture to that of embryos that did not. METHODS: A retrospective cohort study of blastocyst freeze/thaw cycles vitrified on day 5, thawed and transferred after 20-22 h in the culture, between January 2012 and December 2016. RESULTS: A total of 375 patients were included in the analysis. Two hundred twenty-eight embryos graded as good, 87 graded as fair, and 60 graded as poor embryos were transferred. The clinical pregnancy rate (50% vs. 19.5% vs 3.3% p < 0.01) and the ongoing pregnancy rate (38.5% vs. 13.6% vs 1.7% p < 0.01) were higher in cases of good embryo quality compared with fair and poor-quality embryos, respectively. For good embryos, progressing to a better grade during the culture did not change the clinical pregnancy rate (51.3% vs. 46.2% p = NS) or the ongoing pregnancy rate (38.5% vs. 37.5% p = NS). For fair embryos, progressing to a better grade during the culture resulted in a higher clinical pregnancy rate (25.4% vs 9% p = 0.05). CONCLUSIONS: The development of the fair embryos in the culture has a highly positive impact on the pregnancy rate and this factor should be taken into consideration before deciding how many embryos to transfer.